1. Field of the Invention
The present invention generally relates to an optical scanning device, an image forming apparatus and an optical scanning method, and, in particular, to improvements in an optical scanning device acting as a writing optical system of an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, or the like.
2. Description of the Related Art
A digital image forming apparatus includes an optical scanning device. In the apparatus, a laser light emitted from the optical scanning device is applied onto a photosensitive body so that an electrostatic latent image is formed thereon, the electrostatic latent image is developed by a developing device by using toner, and the thus-obtained toner image is transferred and fixed onto a transfer paper. Thereby, image formation is rendered. The optical scanning device is a device which produces the laser light based on an electric image signal obtained from conversion from optical image data such as that of a reflected beam obtained from reading and scanning operation performed by a reading optical system, and emits the produced laser light onto the photosensitive body.
FIG. 1A shows a plan view of an optical scanning device (laser writing device) in the related art, FIG. 1B shows a configuration of an essential part thereof, and FIG. 1C shows a perspective view of an essential part thereof.
In FIGS. 1A and 1B, a beam emitted by an LD unit 1 is condensed in sub-scan directions (directions perpendicular to FIG. 1A) by a cylindrical lens 2. The condensed beam is used to image a latent image long along a main scan direction on or in the vicinity of a deflection reflective surface 4 of a polygon mirror after passing through a transparent parallel plate 3 provided on a window of a housing, not shown in the figure, tightly enclosing the polygon mirror (light deflector) 5.
A reflected beam (deflected beam) from the deflection reflective surface 4 passes through the parallel plate 3, is incident on a lens system part 6 (lens 6′ and lens 6″) having an fθ function and a long-dimensional lens 8, respectively, and, then, is condensed toward a surface of a photosensitive body (surface to be scanned) 7. The lens system part 6 (lens 6′ and lens 6″) having the fθ function and long-dimensional lens 8 act as a scanning and imaging lens.
Recently, high density image formation has been demanded. Accordingly, reduction of diameter of a beam spot formed on the surface to be scanned 7 is strongly demanded. For this purpose, complex shapes such as aspherical surfaces, special toroidal surfaces and so forth have been employed in the fθ lens system part (lenses 6′ and 6″) and long-dimensional lens 8 of the scanning and imaging lens, and these lenses have been made of plastic through resin molding technique in many cases.
A lens made of plastic (referred simply to as ‘plastic-made lens’, hereinafter) has its rigidity increased as a result of a lens body thereof (long-dimensional lens 8) being supported by a holding frame (rib) 9, and, thereby, deformation thereof due to time elapsing is avoided. Specifically, manufacture is made such that the holding frame 9 is formed integrally with the long-dimensional lens 8 so as to enclose the four sides of the lens 8. As shown in FIGS. 1B and 1C, rib surfaces 9a which are parallel to sub-scan directions of the long-dimensional lens 8 are formed on the inner surfaces at both ends in the longitudinal directions of the holding frame 9.
The holding frame 9 is provided outside of an effective diameter (a range of the lens surface corresponding to the range through which a proper image is written on the photosensitive body which is an effective image display device writing region/range). However, the beam needs to be made to pass through the lens even outside of the effective diameter for the purpose of synchronization of each scan line. Specifically, a synchronization detecting sensor, not shown in the figure, is provided outside of the effective diameter in a main scan direction, and, thereby, a first scanning point of the deflected beam and scanning speed (time) are detected.
At this time, as also shown in FIG. 1B, the beam B applied to the outside of the effective diameter is reflected by the rib surface 9a so as to become a ghost light, is then applied to the surface to be scanned 7, and, as a result, an abnormal image may formed thereon. This is because, as the rib surfaces 9a, in the related art, are surfaces parallel to the sub-scan directions of the long-dimensional lens 8, the ghost light therefrom is applied to the surface to be scanned 7 within an effective image region thereof when the beam B applied to the outside of the effective diameter is reflected by the rib surface 9a, and, thereby, an abnormal image is formed on the surface to be scanned 7, in many cases.